[R] Unwanted white borders on semi-transparent polygons?

[Matthew Neilson]

Thanks for your fast response.

I'm using R version 2.1.1 on OS X 10.3.9 to create the pdfs. I have tried viewing the pdf output in both Acrobat 6 and 7 (both display a white border around each polygon) as well as 
Preview (displays fine). I have emailed the pdf file to some correspondents running Windows, and they also see white borders when viewing with Acrobat (version unspecified).

I have tried using R version 2.4.0 on a G5 machine (which I can access remotely) running OS X 10.4.8, but the resulting pdf renders incorrectly (i.e. with a white border around each 
polygon) in both Acrobat *and* Preview. So it would appear that the combination of R 2.1.1 and OS X 10.3.9 gives slightly better results - although plots still appear incorrect when 
printed or viewed in Acrobat.

Unfortunately, I don't have access to a Windows machine to test this out. Even if I did, many of my scripts include various Unix system calls so I don't think that would be a viable 
solution. Could this be a bug in the OS X pdf driver?

Many thanks,


-Matt



On 27 Apr 2007, at 06:42, Prof Brian Ripley wrote:

>What version of R, what OS, what version of Acrobat?
>
>I don't see this in 2.5.0 on Windows (using Acrobat 7: Acrobat does not 
>exist on Linux, AFAIK).  And reading the PDF produced shows no sign of an 
>extra object for the border.
>
>On Fri, 27 Apr 2007, Matthew Neilson wrote:
>
>> Hey all,
>>
>> I'm trying to create a plot of two semi-transparent regions. The reason they need to be partially transparent is so that I can see if there's any overlap. Here's some example code:
>>
>> # BEGIN
>>
>> pdf(file="test.pdf",version="1.4")
>> plot(0,0,type="l",ylim=range(-3,3),xlim=range(-1,5))
>> polygon(c(0,1,2,3,4,3,2,1,0), c(0,1,2,1,0,-1,-2,-1,0), col=rgb(1,0,0,0.5),
>> border=NA)
>> polygon(c(1,2,3,4,5,4,3,2,1), c(0,1,2,1,0,-1,-2,-1,0), col=rgb(0,0,1,0.5),
>> border=NA)
>> dev.off()
>>
>> # END
>>
>> The problem with this is that, despite setting "border = NA", I get a 
>> big white border surrounding each polygon!! Funnily enough, setting the 
>> alpha channel equal to 1 (as opposed to 0.5) *doesn't* give the border, 
>> but an alpha channel of 1 produces an opaque polygon! :S
>>
>> I have read the FAQ, and (unfortunately) turning off line-art smoothing 
>> does not give the desired effect. Furthermore, my pdfs print with a 
>> white border surrounding each transparent polygon.
>>
>> Now, here comes the really bizarre part. Whilst Adobe Acrobat displays 
>> the unwanted white border, Apple Preview respects the "border=NA" 
>> argument and shows the two diamonds as they are intended. However, 
>> opening up the pdf in Illustrator CS reveals that there is in fact a 
>> transparent (according to Illustrator) border *on top* of each diamond. 
>> Deleting these two borders (one for each polygon) and re-saving the pdf 
>> appears to correct the issue. So the obvious question is: how did the 
>> surrounding borders get there in the first place? A bug in the polygon 
>> function, perhaps?
>>
>> Does anyone have any ideas for preventing these unwanted borders around 
>> semi-transparent polygons (without having to resort to Illustrator)? Has 
>> anyone else even come across this problem?
>>
>> Many thanks,
>>
>>
>> -Matt
>>
>
>-- 
>Brian D. Ripley,                  ripley at stats.ox.ac.uk
>Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
>University of Oxford,             Tel:  +44 1865 272861 (self)
>1 South Parks Road,                     +44 1865 272866 (PA)
>Oxford OX1 3TG, UK                Fax:  +44 1865 272595